1. Field of the Invention
The present invention relates to foamed well cement slurries, additives for foaming and stabilizing the foamed cement slurries and methods of using the slurries.
2. Description of the Prior Art
Hydraulic cement slurries are commonly utilized in subterranean well completion and remedial operations. For example, hydraulic cement slurries are used in primary cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement slurry is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement slurry is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the exterior surfaces of the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In carrying out primary cementing as well as remedial cementing operations in well bores, the cement slurries utilized must often be light weight to prevent excessive hydrostatic pressure from being exerted on subterranean formations penetrated by the well bore. As a result, a variety of light weight cement slurries have heretofore been developed and used including foamed cement slurries.
In addition to being light weight, a foamed cement slurry contains compressed gas which improves the ability of the slurry to maintain pressure and prevent the flow of formation fluids into and through the slurry during its transition time, i.e., the time during which the cement slurry changes from a true fluid to a hard set mass. Foamed cement slurries are also advantageous because they have low fluid loss properties.
While the heretofore used foamed cement slurries have included various surfactants known as foaming agents for facilitating the foaming of cement slurries when a gas is mixed therewith and various other surfactants known as foam stabilizers for preventing the foamed cement slurries from prematurely separating into their slurry and gas components, various problems in the use of such surfactants have been experienced.
For example, when saltwater is utilized for forming a cement slurry and the saltwater contains salt in an amount above about 18% by weight up to the saturation amount, it has often heretofore been necessary to utilize a particular foaming agent and a particular foam stabilizer in order to produce a foamed cement slurry having a relatively short set time. For example, U.S. Pat. No. 5,147,565 issued to Bour et al. on Sep. 15, 1992 discloses foamed hydraulic cement slurries containing saltwater, a foaming agent consisting of a surfactant of the formula R--(OR').sub.n --OSO.sub.3 --X.sup.+ and a separate foam stabilizer such as a glycol of the formula CH.sub.3 O--(CH.sub.2 CH.sub.2 O).sub.n H or a betaine surfactant. While the foamed cement slurries described in the patent are suitable for carrying out operations in salt containing subterranean formations and are relatively quick setting, the presence of the above described foaming agent and foam stabilizer in the foamed cement slurries lowers the compressive strengths of the slurries after they set.
More recently, an improved single universal additive for foaming and stabilizing a cement slurry containing fresh water or saltwater has been developed and used and is the subject matter of my co-pending patent application Ser. No. 08/899,043 filed on Jul. 23, 1997 now U.S. Pat. No. 5,897,699. The additive for forming and stabilizing a cement slurry described and claimed therein is comprised of about 2 parts by weight of an alpha-olefinic sulfonate surfactant having the formula EQU H(CH.sub.2).sub.n --CH.dbd.CH--(CH.sub.2).sub.m SO.sub.3 Na
wherein n and m are individually integers in the range of from about 6 to 16, and about 1 part by weight of a betaine surfactant having the formula EQU R--CONHCH.sub.2 CH.sub.2 CH.sub.2 N.sup.+ (CH3).sub.2 CH.sub.2 CO.sub.2.sup.-
wherein R is a radical selected from the group of decyl, cetyl, oleyl, lauryl and cocoyl.
While the above described additive is effective in foaming and stabilizing cement slurries, there is a continuing need for improved universal foaming and stabilizing additives for cement slurries which are effective in well cement compositions containing various types of cement and water and which form very stable foamed cement slurries at low concentrations.